Opening Up New Possibilities for Wooden Construction
Development of a Wood-Steel Hybrid Frame System
New Construction Methods for the Decarbonization Era
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Nikken Sekkei has jointly developed a “wood-steel hybrid rigid-frame structure” that makes extensive use of wood in its structure, in collaboration with Nippon Steel Engineering. Developed under the guidance of Professors Toru Takeuchi and Hiroyasu Sakata of Tokyo Institute of Technology, this rigid-frame structure—which combines steel columns with wooden beams—received structural performance certification in September 2020. It will be widely applied to medium- to large-scale buildings such as mid-rise buildings, hotels, and schools, and will contribute to decarbonization and environmental issues by creating aesthetically pleasing wooden spaces.
Moving Toward the Development of a Hybrid That Compensates for Wood's Fragility with Steel
“Is it possible to develop a new construction method—one that is simple, easy to implement, and highly versatile—to create large spaces that showcase the beauty of wood?” The “Wood-Steel Hybrid Rafter Frame” project began with this idea from structural engineer Kimiaki Harada.
Nikken Sekkei has undertaken numerous wood-based and wood-composite projects aimed at expanding the use of wood to help combat global warming and preserve satoyama landscapes.Notable examples include “W350 Plan,” which envisioned a wooden skyscraper; “Mokuzai Kaikan,” which made full use of wood in its structure and exterior cladding; and “Ariake Gymnastics Centre”*, which realized a large-scale wooden space using Japan’s first composite timber-tension-beam structure.
Nikken Sekkei has undertaken numerous wood-based and wood-composite projects aimed at expanding the use of wood to help combat global warming and preserve satoyama landscapes.Notable examples include “W350 Plan,” which envisioned a wooden skyscraper; “Mokuzai Kaikan,” which made full use of wood in its structure and exterior cladding; and “Ariake Gymnastics Centre”*, which realized a large-scale wooden space using Japan’s first composite timber-tension-beam structure.
W350 Plan © Sumitomo Forestry and Nikken Sekkei
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Mokuzai Kaikan © Nacása & Partners Inc.
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Ariake Gymnastics Centre © SS Co.,Ltd.
Through these experiences, the concept of a hybrid structure—which uses steel to reinforce the joints, a weak point of wood—emerged as a way to expand the use of wood-based structures in mainstream architecture toward a sustainable society.While there had been examples of wood-and-steel hybrid structures in the past, they required complex construction processes, such as the use of a large number of bolts. “To popularize wood-based structures, we cannot make a breakthrough unless we establish a simple construction method that can be used for a variety of purposes,” says Harada, explaining the background behind their research and development efforts.
In 2016, he recruited a small group of volunteers within the company who were interested in the new construction method, and research and development began with just a few people.When they consulted Professor Toru Takeuchi at Tokyo Institute of Technology—who researches steel structures, seismic-resistant structures, and spatial structures—they were assigned to work under his guidance alongside Professor Hiroyasu Sakata, a specialist in concrete and timber structures at the same university. Nippon Steel Engineering, which possesses advanced technical expertise and know-how in steel structures, also joined the effort, forming a joint development team.
Initially, we began by exploring wood-steel connections for roofs spanning large spaces.Team members each drew sketches and shared their ideas. We explored various proposals, including bolting laminated timber to both sides of a T-shaped steel section, inserting laminated timber into an H-shaped steel beam, and a steel unit with an integrated bolt and joint capable of accommodating various angles.
In 2016, he recruited a small group of volunteers within the company who were interested in the new construction method, and research and development began with just a few people.When they consulted Professor Toru Takeuchi at Tokyo Institute of Technology—who researches steel structures, seismic-resistant structures, and spatial structures—they were assigned to work under his guidance alongside Professor Hiroyasu Sakata, a specialist in concrete and timber structures at the same university. Nippon Steel Engineering, which possesses advanced technical expertise and know-how in steel structures, also joined the effort, forming a joint development team.
Initially, we began by exploring wood-steel connections for roofs spanning large spaces.Team members each drew sketches and shared their ideas. We explored various proposals, including bolting laminated timber to both sides of a T-shaped steel section, inserting laminated timber into an H-shaped steel beam, and a steel unit with an integrated bolt and joint capable of accommodating various angles.
Sketch
We then decided to conduct full-scale experiments to ensure the safety of this entirely new joint design. We created several mockups—including a robust design that reinforces the top and bottom of laminated timber with steel plates, and a design that uses T-shaped steel sections to prioritize aesthetics, making the structure appear to be all-wood when viewed from below—and repeatedly conducted destructive tests under bending stress. As a result, it was found that the method of securing the outermost layer of wood with steel plates produced a joint that was stronger than expected. While joints in all-wood construction using standard metal fittings typically have a strength of about 30% of the wood’s bending strength, this method was confirmed to have a strength of nearly 90%.
Experiments on Wood-Steel Hybrid Joints in Timber Space Structures
Structural Experiments at Tokyo Institute of Technology
“In that case, perhaps we could use engineered wood as beams not only for roofs but also in moment-resisting frame structures—” It was in 2018, two years after the research began, that the R&D team discovered the potential of hybrid moment-resisting frame structures with columns and beams—a type in higher demand than those for large open spaces.
"Dogbone," a design born on the West Coast, enables earthquake-resistant joints
A wood-steel hybrid frame consisting of steel columns and wooden beams. Although the structure appears surprisingly simple—so much so that it’s hard to believe it hasn’t existed before—we were able to arrive at a design that brings out the best in both steel and engineered wood by conducting calculations and analyses in parallel, destroying dozens of full-scale test specimens, and verifying performance along the way.
Axonometic Projection of the Wood-Steel Hybrid Ramen Frame
In the roof of the large space, the beam depth was about 300 mm, but to convert it to a rigid-frame structure, the beam depth increased to a maximum of 1,200 mm, resulting in larger structural members. However, even so, using wood at the ends of the beams makes the structure more brittle than a steel-frame structure, and it would break with a cracking sound during a strong earthquake. To address this, a design was devised to shift the seismic load onto the steel—commonly known as the “dogbone”—which features slightly smaller beam sections at the ends. This design was inspired by the “Reduced Beam Section” method of steel construction, which gained widespread adoption following the 1994 Northridge earthquake on the U.S. West Coast. When a building deforms during an earthquake, concentrating the stress in this “dogbone” section allows the steel to undergo flexible plastic deformation, thereby increasing the building’s ductility.
Axometric Projection: Details of the Joints
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Components of the Wood-Steel Hybrid Frame
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Design Principles for Wood-Steel Hybrid Ramen Structures
As a result, we have succeeded in creating joints that are resistant to bending—something that had been difficult to achieve with conventional timber structures. It took five years from the start of the research. Because wood is not a homogeneous material, we created three identical test specimens each time and conducted full-scale failure tests on several types of joints. The total number of test specimens amounted to more than 60. Mizuki Shigematsu, who joined the team in 2018 and was in charge of structural analysis and other tasks, reflects on the research and development process, saying, “When tackling a new structure, I realized the importance of verifying it with physical specimens, not just relying on theoretical calculations.”
Photos from the experiment
Experiment on Wood-Steel Hybrid Ramen Structures
Structural Experiments at Tokyo Institute of Technology
Toward the Expansion of Environmentally Friendly Wooden Spaces Designed for Their Intended Purposes
In 2020, the wood-steel hybrid frame system finally received a structural performance evaluation from Japan ERI. This evaluation is intended to facilitate the more widespread construction of “buildings using special structural methods” that are not specifically regulated by the Building Standards Act. With this evaluation in place, the system can be applied to future construction projects simply by filing a building permit application—without the need for complex procedures—which will lead to its wider adoption in the future.
With an eye toward future developments, we are also conducting preliminary cost-related design studies based on a hypothetical five-story hotel.
Compared to all-wood construction, this system is more cost-effective and allows for greater design flexibility without the need for plywood walls or diagonal bracing.Compared to reinforced concrete or steel-frame construction, the total construction cost was slightly higher at this stage. However, since the use of wood reduces the building’s weight, foundation costs may be lower depending on site conditions. Additionally, exposing the wooden beams eliminates the need for interior finishing, which helps shorten the construction schedule.
The wood-steel hybrid frame emits only about 40 percent of the CO₂ emitted during the manufacturing of reinforced concrete structures, and furthermore, most of the CO₂ emitted can be sequestered within the wood itself. In the decarbonized society of the future, the use of wood in construction will offer significant benefits.
Aiming to create “a structure that is gentle on both its users and the global environment,” Rinko Mori joined the team and conducted environmental impact assessments.
This hybrid system is a rational design that maximizes the environmental benefits of wood while ensuring the necessary strength by using steel in columns and joints. “Rather than using steel simply to incorporate wood, this is a new structural form that puts the right material in the right place.I believe we were able to propose a system that makes efficient use of materials,” reflects Miwa Sadayuki, who participated as a structural engineer from the project’s inception.
“It looks good, and it’s strong and reliable” (Harada)—the wood-steel hybrid frame. In addition to the column-and-beam structure, the team aims to obtain performance evaluations for the wooden spatial structure covering large open spaces, which was initially under consideration. They continue their relentless pursuit of making public spaces—such as atriums and station buildings—more comfortable and stronger.
With an eye toward future developments, we are also conducting preliminary cost-related design studies based on a hypothetical five-story hotel.
Compared to all-wood construction, this system is more cost-effective and allows for greater design flexibility without the need for plywood walls or diagonal bracing.Compared to reinforced concrete or steel-frame construction, the total construction cost was slightly higher at this stage. However, since the use of wood reduces the building’s weight, foundation costs may be lower depending on site conditions. Additionally, exposing the wooden beams eliminates the need for interior finishing, which helps shorten the construction schedule.
The wood-steel hybrid frame emits only about 40 percent of the CO₂ emitted during the manufacturing of reinforced concrete structures, and furthermore, most of the CO₂ emitted can be sequestered within the wood itself. In the decarbonized society of the future, the use of wood in construction will offer significant benefits.
Aiming to create “a structure that is gentle on both its users and the global environment,” Rinko Mori joined the team and conducted environmental impact assessments.
This hybrid system is a rational design that maximizes the environmental benefits of wood while ensuring the necessary strength by using steel in columns and joints. “Rather than using steel simply to incorporate wood, this is a new structural form that puts the right material in the right place.I believe we were able to propose a system that makes efficient use of materials,” reflects Miwa Sadayuki, who participated as a structural engineer from the project’s inception.
“It looks good, and it’s strong and reliable” (Harada)—the wood-steel hybrid frame. In addition to the column-and-beam structure, the team aims to obtain performance evaluations for the wooden spatial structure covering large open spaces, which was initially under consideration. They continue their relentless pursuit of making public spaces—such as atriums and station buildings—more comfortable and stronger.
Axonometric Projection of a Wooden Spatial Structure
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Single-Layer Lattice Shell Roof
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Axonometric View of a Wood-Steel Hybrid Joint
| ※ | Basic Design, Supervision of Detailed Design, and Construction Supervision | ： | Nikken Sekkei Ltd |
| Detailed Design | ： | Shimizu Corporation, Kimio Saito (Technical Advisor) |
